THE SPECTRUM OF IN-VITRO RADIOSENSITIVITY IN 4 HUMAN-MELANOMA CELL-LINES IS NOT ACCOUNTED FOR BY DIFFERENTIAL INDUCTION OR REJOINING OF DNADOUBLE-STRAND BREAKS

Purpose: Radioresistance is a significant clinical problem in advanced malignant melanoma and many melanoma cell lines show a radioresistant acute x-ray survival response in vitro. Given that the DNA double str and break is the lesion most closely correlated with x-ray induced cel l lethality, differences in the induction and rejoining of these lesio ns may account for the radioresistance of some human melanoma cell lin es. Methods and Materials: The above hypothesis was tested using pulse d field gel electrophoresis to measure x-ray induced DNA double strand break induction and rejoining in four human melanoma cell lines: MM13 8, MM170, MM96-L and HT 144.Results: The MM138, MM170 and MM96-L cell lines were characterized in vitro by low alpha/beta ratios and broad x -ray survival curve shoulders. MM138 and MM170 were the most radioresi stant and MM96-L had intermediate sensitivity. In contrast, HT144 was markedly x-ray sensitive, despite retaining a shoulder and like the ot her lines, having a low alpha/beta ratio. There were no significiant d ifferences in DNA double strand break induction between the cell lines , and thus no correlation existed between DNA double strand break indu ction and radiosensitivity. Consistent with the shoulders on the g-ray survival curves, all four cell lines shelved efficient DNA double str and break rejoining. Highly efficient DNA double strand break rejoinin g could account for the radioresistance of one of the melanoma lines ( MM138). For example, MM138 had rejoined 50% of the induced DNA double strand breaks by 5.5 min compared to 13-17 min for the other three cel l lines. The development of postirradiation apoptosis was effectively excluded as the cause of the marked radiosensitivity of the HT144 cell line. Conclusion: Other factors (such as lesion repair fidelity or di fferential lesion tolerance) underlie the differences in the intrinsic radiosensitivity between these melanoma cell lines.